Method and apparatus for determining radiation transmission characteristics of a generally transparent medium

ABSTRACT

A device for determining the spectral radiation transmission characteristics and size-frequency distribution of very small particles dispersed in a generally transparent liquid medium comprises a transparent sampling cell through which the liquid medium is moved at a desired rate and subjected to a beam of radiation by a suitable optical system. The optical system, which includes a rotatable, spectrally variable filter and a suitable mask limits the beam of radiation incident on one side of the cell to a narrow band, the peak wavelength of which is a function of angular displacement. On the opposite side of the sampling cell, a plurality of photosensitive detectors are arranged for generating an electrical signal from the radiation incident thereon that is in accordance with the amount of radiation emitted from the cell at various angles relative to the axis of the beam of radiation. A photosensitive detector is also arranged relative to the one side of the cell and the transmittance side of the filter to detect the radiation incident on the one side of the cell so as to provide a reference signal for relative measurement of the radiation emitted by the cell on the other side thereof. The signals derived from the plurality of detectors and from the reference detector are amplified and by suitable circuitry generate an analog output signal that corresponds to the spectral transmission characteristics of the liquid medium moving through the cell.

United States Patent [19'] Seelbinder 3,724,95 1 Apr. 3 1973 Inventor:

[75] David C. Seelbinder, Rochester,

[73] Assignee: Eastman Kodak Company, Rochester, N.Y.

Filed: Aug. 31, 1971 Appl. No.2 176,492

[56] References Cited UNITED STATES PATENTS 2/ 1972 4/1969 9/1971 4/197010/1967 Martens et al.. 1/1960 Appel et al.

OTHER PUBLICATIONS Particle Size Determination by Turbidimet ry, R. V.Gledhill,.lr., Physical Chemistry, 66, 458 (1962).

Beattic et a1. ..356/104 Rudomanski et al ..356/89 PrimaryExaminen-Ronald L. Wibert Assistant ExaminerU. P. McGraw Attorney-RobertW. Hampton et a1.

[57] ABSTRACT A device for determining the spectral radiationtransmission characteristics and size-frequency distribution of verysmall particles dispersed in a generally transparent liquid mediumcomprises a transparent sampling cell through which the liquid medium ismoved at a desired rate and subjected to a beam of radiation by asuitable optical system. The optical system, which includes a rotatable,spectrally variable filter and a suitable mask limits the beam ofradiation incident on one side of the cell to a narrow band, the peakwavelength of which is a function of angular displacement. On theopposite side of the sampling cell, a plurality of photosensitivedetectors are arranged for generating an electrical signal from theradiation incident thereon that is in accordance with the amount ofradiation emitted from the cell at various angles relative to the axisof the beam of radiation. A photosensitive detector is also arrangedrelative to the one side of the cell and the transmittance side of thefilter to detect the radiation incident on the one side of the cell soas to provide a reference signal for relative measurement of theradiation emitted by the cell on the other side thereof. The signalsderived from the plurality of detectors and from the reference detectorare amplified and by suitable circuitry generate an analog output signalthat corresponds to the spectral transmission characteristics of theliquid medium moving through the cell.

15 Claims, 4 Drawing Figures PATENTEUAPRS I975 3.724.951

SHEET 1 OF 2 ANALOG OUTPUT D/SPLA) S/ZE-FREOUENCY PLOT k COMPUTER T0L/GHT DE TE CTORS 4048 F762 4 Burg 4 AGE/VT 1 METHOD AND APPARATUS FORDETE" :l G RADIATION TRANSMISSION CHARACTERISTICS OF A GENERALLYTRANSPARENT MEDIUM FIELD OF THE INVENTION The invention relates to amethod and apparatus for determining the radiation transmissioncharacteristics of very small particles dispersed in a generallytransparent liquid medium and, more particularly, to a method andapparatus by which such characteristics can be related to particle sizedistribution and rapid changes in such distribution as a function oftime.

DESCRIPTION OF THE PRIOR ART it is well-known in the art that the lightscattering property or characteristic of small particles is a functionof their size. The light scattering properties of unexposed andundeveloped photographic emulsions are used as a basis for empiricalprediction of the sizefrequency distribution of the silver halide grainsin the emulsions. The diameter of small spherical particles can becomputed from sample spectral transmittance measurements, as describedin the article entitled PAR- TICLE-SIZE DISTRIBUTION DETERMINATION BYTURBIDIMETRY, by R. V. Gledhill, Jr., Physical Chemistry, 66, 458(1962). The size-frequency dis tribution of small non-sphericalparticles can also be determined by X-ray microscopy measurement orindirectly by spectrogoniophotometric analysis. Such analysis made withrespect to a wide variety of emul sion coatings containing specifiedamounts of silver and gelatin have been functionally related tograin-size data obtained from photomicrographic measurements of emulsionsmears. However, while these methods of measuring grain-sizedistribution have been useful in emulsion research and .technology,their application has been limited because of the time required toobtain and correlate the data. As a result, these methods, which arewell-known to those skilled in the art, do not readily lend themselvesto a commercial, in-line production process for a liquid medium, inwhich such measurements must be made to maintain control of the processand subsequent manufacturing operations.

SUMMARY OF THE INVENTION One object of the invention is to provide amethod and apparatus-for determining the size-frequency distribution ofvery small particles dispersed in a generally transparent liquid medium.

Another object of the invention is to provide a method and apparatus bywhich the size-frequency distribution of very small particles in agenerally transparent liquid medium can be determined in an inline"manufacturing operation for the liquid medium containing such particles.

Still another object of the invention is a method and apparatus by whichthe size-frequency distribution of very small particles in a generallytransparent liquid medium is useful to control the formulation processfor the liquid medium, as well as subsequent manufacturing operations.

Yet another object of the invention is to provide a method and apparatusfor determining the sizefrequency distribution of very small particlesdispersed in a generally transparent-liquid medium wherein suchdistribution is undergoing a rapid change, such as occurs in silverhalide grains during the formation of photographic emulsionprecipitation and ripening processes.

These and other objects andadvantages of the invention will be apparentto those skilled in the art by the description which follows.

The method and apparatus for attaining the abovementioned objects areconsidered to be unique in that the size-frequency distribution of verysmall particles dispersed in a generally transparent liquid medium canbe measured while they are undergoing a rapid change. Such measurementshave been found to be very useful in the control of processes andsubsequent manufacturing operations, particularly with respect to theproduction of photographic emulsions.

The objects of the invention are attained by a device comprising atransparent sampling cell through which a generally transparent liquidmedium, such as a photographic emulsion which has particles dispersedtherein, can flow at a desired rate. A source providing a beam ofradiation is directed along an axis generally normal to one side of thecell by a suitable optical system. A rotatable, spectrally variablefilter is arranged intermediate the source and the sampling cell. Thefilter transmits a narrow band of radiation, the peak wavelength ofwhich is a function of angular displacement. 0n the opposite side of thesampling cell, a plurality of lenses are arranged in a generally arcuateconfiguration relative to the cell, each lens being associated with aphotosensitive detector for generating an electrical signal from theradiation incident thereon that is in accordance with the amount ofradiation emitted from the cell at various angles relative to the axisof the beam of radiation. A photosensitive detector is also arrangedrelative to the one side of the cell and the transmittance side of thefilter to detect the radiation incident on the one side of the cell soas to provide a reference signal for relative measurement of theradiation emitted by the cell on the other side thereof. Suitableindicia or marks are provided on the filter for controlling theinitiation of a recorder and for indicating the transmittal ofpreselected wavelengths of radiation. The signals derived from theradiation transmittance detectors and from the radiation referencedetector are amplified and by suitable circuitry generate an analogoutput signal that corresponds to the spectral transmissioncharacteristic of the liquid medium moving through the cell. This signalis related to wavelength and a visual presentation of this relationshipis displayed by the recorder. By relating the spectral transmissionsignals to empirical information programmed in a computer, an output canbe obtained from the computer which will provide a particle size--frequency distribution plot. The data can be used to ob tain informationwhich relates the changes in particle size-frequency distribution as afunction of time.

DESCRWIION OF THE DRAWINGS FIG. 1 is a schematic, perspective viewshowing the.

relationship of the various elements comprising the device fordetermining the transmission radiation characteristics of a generallytransparent liquid medium;

FIG. 2 is a schematic view showing the circuitry in block form inconjunction with the device to provide a visual display of the particlesize-frequency distribution and spectral transmission characteristics asaccomplished by the device generally disclosed in FIG. 1;

FIG. 3 is a partial perspective view showing the manner in whichphotosensitive detectors associated with a rotatable filter of variablespectral transmittance are used to control operation of a recorder andto designate periodically the wavelength of the radiation transmitted bythe filter; and

FIG. 4 is a block diagram showing the manner in which the signalsgenerated by the photosensitive detectors can be used to provide ananalog output display and a size-frequency distribution display.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference particularly toFIG. 1, a generally transparentv liquid medium, such as a photographicemulsion, can be contained in a receptacle which is connected to asample cell 19 by suitable piping and a pump 11. The pump 11 moves theemulsion at a desired rate, which may be uniform or variable andcontinuous or intermittent, through a flow path including the samplecell 19. This cell is provided with transparent side plates so theliquid medium can be readily exposed to radiation. A source of radiationwhich emits over a broad spectral band including ultraviolet (UV) andinfrared (IR) can be used. In the disclosed embodiment, a tungsten lamp21 is used and has a lens 22 associated therewith for collimating theradiation emitted by lamp 2] along an axis that is normal to the one(incident) side of cell 19.

A rotating, spectrally variable filter 23, which transmits a narrow bandwidth of radiation, the wavelength of which is a function of angulardisplacement, is arranged between the source of radiation 21 and thecell 19 and is driven at a continuous, uniform rate by a motor 25. Apair of adjustable plates 24 are mounted relan've to the optical axis offilter 23 and form a slit 12 for controlling the bandwidth of radiationincident on the cell 19 and the liquid medium therein.

A lens 26 and a photosensitive detector 27 are optically aligned andarranged so the beam of radiation incident on the cell 19 will also beincident on lens 26 which focuses the radiation on detector 27 so as togenerate an electrical signal corresponding thereto that will serve as areference signal with respect to the signals derived from the radiationincident on the cell and transmitted by the liquid medium as describedmore fully hereinafter. A field lens 28 is arranged adjacent the oneside of the cell 19 facing the filter 23 to control and limit the spreadof the beam of radiation incident on the cell 19.

On the opposite side of cell 19, a plurality of lenses 30 andphotosensitive detectors 31 are aligned in sets, that is, each of lenses30 is associated with a respective detector 31, each set being arrangedto intercept and detect radiation transmitted by the liquid mediumflowing through cell 19, the radiation being emitted by cell 19 atvarious angles relative to the axis of the beam of radiation that isdirected onto the cell 19 via lamp 21,

lens 22, filter 23, and lens 28. One of the lenses 30 and one of thedetectors 31 are arranged relative to the axis per 'se with respect towhich the lenses 22 and 28 and the lamp 21 are also aligned, suchdetector providing an electrical signal which will be indicative of theradiation incident on cell 19 and transmitted with substantially zeroscatter.

The output signals from the detectors include a number of variables thatmust be taken into consideration if a true determination of theradiation transmission characteristics of the liquid medium is to bemade. All of the variables will be sensed by the detectors 31 so itbecomes necessary to provide a reference with respect to the scatteredor emitted radiation. For this reason, detector 27 is placed in thesystem. The variables that must be considered are: (l) the spectralemittance of the source of radiation (lamp 21), i.e. at differentwavelengths variable amounts of energy are released; (2) the spectraltransmittance of the filter 23, i.e. at different wavelength bandsvariable fractions of incident energy are transmitted; and (3) thespectral efficiency of the detectors 31 per se, i.e. for each wavelengtha given amount-of energy incident on a detector will produce a differentlevel of output signal. When these factors are considered, thedifference between the signal generated by detector 27 and thatgenerated by each of cells 31 will then be a true and accurateindication of the relative spectral radiation transmitted or emitted bythe liquid medium on axis and at various angles and, hence, its spectraltransmission (spectrogoniophotometric) characteristics.

Hence, the electrical signal generated by the detector 27 will bederived from the radiation from lamp 21 transmitted by the filter 23 aswell as the spectral sensitivity of the detector 27 per se. This signalis amplified by a logarithmic amplifier 40 and transmitted to adifferential amplifier 41, see FIG. 2. Also, this same signal will betransmitted to an intensity modulator 42 which includes a loop controlfor maintaining the signal generated by detector 27 at a generallyuniform level to minimize the total signal range generated by detectors31 that must be logarithmically amplified.

It is to be understood, as the filter 23 is rotated, the radiationincident on cell 19 is continuously changing. This change is withrespect to the spectral characteristics of the radiation; and, as such,the reflection or diffraction of this light by the particles dispersedin the liquid medium will provide different angles of scatter to theradiation which will be picked up by each of the detectors 31 as theyare individually and sequentially rendered operative. As shown in FIG.2, the detectors 31 are controlled so as to be sequentially operated bymeans of switching circuit 45. An automatic scanning circuit 47 isprovided for controlling the operation of circuit 45. During normaloperation, scanning circuit 47 will provide for automatic and sequentialoperation of the detectors 31 so that a series of electrical signals aregenerated, each in accordance with the radiation transmitted through theliquid medium and scattered by the particles, as detected from aparticular angle by the energized detector 31. The signal generated byeach of detectors 31 is amplified by a logarithmic amplifier 48 andtransmitted to the differential amplifier 41. The two signals, the onefrom the amplifier 40 (detector 27) and the other from the amplifier 48(energized detector 31) are differentiated by amplifier 41 and a seriesof output signals (analog data) is obtained which correspond to thespectral transmission density characteristic of the liquid medium onaxis and at various scatter angles. This series of signals can then betransmitted to an X-Y recorder 50 which will provide a visual displaywhich relates the transmission characteristics to wavelengths ofradiation at various scatter angles.

With reference to FIG. 3, the filter 23 is calibrated with indicia so asto provide a signal which controls initiation of the X-Y recorder 50, asignal which initiates scanning circuit 47, and a series of signalscorresponding to wavelength designations which are related to theoptical characteristics of filter 23. The latter series of signals isalso used with reference to the display indicated by 55 in FIG. 4. Thevariable spectral transmission of light by filter 23 is carefullymeasured; and at several points, for example, those equivalent to 4,000,5,000, 6,000, and 7,000 Angstrom units, an aperture 56 is placed in thefilter adjacent its peripheral edge as shown in FIG. 3. A light source57 is arranged on one side of the filter and a photodetector 58 isaligned with the light source and on the opposite side of the filter. Inaddition, an aperture 59 and an aperture 63 are provided, which inrelation to the apertures 56, are arranged such that a signal will bederived at the proper time to first initiate the auto-scanning circuit47 and then to initiate the recorder start circuit 60, the apertures 59and 63 being spaced radially inward or outward relative to the apertures56, see FIG. 3. A detector 61 and a detector 64 is also associated witheach respective aperture 59 and 63, as well as a light source 71 and,72. It is to be understood that the signals derived from apertures 56,59 and 63 can be generated by means other than that just described. Forexample, magnetic areas can be placed on filter 23 in accordance withthe apertures sensed by a pick-up head. Also, the apertures per se canbe replaced with transparent or opaque areas on filter 23 and sensed byphotocells.

Other means can also be devised to generate the necessary signals. Thedisplay derived from the apertures 56 is indicated by the numeral 62 inFIG. 4 and comprises a series of blips, each representative of theparticular wavelength designated by the respective aperture 56.

, While a switch 65 is shown in FIG. 2 as being movable between aposition in which the signals from amplifier 41 are connected torecorder 50 or the signals from detector 58 are connected through areference circuit 66 to recorder 50, the two sets of signals can be usedas inputs when the recorder 50 is one that can produce more than onedisplay as shown in FIG. 4.- Otherwise, the wavelength display 62 willhave to be recorded be fore or after the transmission display 55. I

Empirical formulas with measured data and the analog data from amplifier41 can, through the medium of a computer 69, provide a basis forobtaining a sizefrequency plot of the particles as shown in FIG. 4 anddesignated by the numeral 68. As a result, data can be obtained whichrelates the spectral transmission characteristics to particle sizefrequency distribution.

While the detectors 27 and 31 have been generally described asphotosensitive detectors, other types of detectors, such as aphotomultiplier tube, can be used as well. Also, detectors 31 have beendescribed as being arranged in a generally arcuate fonnat with respectto cell 19, see FIG. 2; however, they can be arranged in any manner solong as an angular displacement is obtained. The arrangement of the lens26 and detector 27 may also take another format to provide the samedegree of sensing control. It should be evident to those skilled in theart that variations, such as those described hereinabove, can be madeand will in no way afiect the function of the method or device nor theresults obtained. t

This invention has been described in detail with particular reference tothe preferred embodiments thereof, but it will be understood thatvariations and modifications may be effected within the spirit and scopeof the invention.

I claim: 1. A method of determining the spectral radiation transmissioncharacteristics and size-frequency dis tributionof very small particlesdispersed in a generally transparent, liquid medium, the stepscomprising:

flowing the liquid medium at a desired rate past a source of radiationhaving broad spectral characteristics; v

continuously varying the spectral characteristics of a narrow bandwidthof the radiation incident on the liquid medium;

generating an electrical reference signal of generally uniform amplitudefrom the radiation of narrow bandwidth;

detecting continuously at a number of different positions relative tothe liquid medium a change in intensity of the transmitted radiation asthe spectral characteristics of the narrow bandwidth varies; generatingsequentially with respect to each different position an electricalsignal corresponding to the change in the transmitted radiationthroughout the variation in the narrow bandwidth spectral charac--teristics; and differentiating the electrical reference signal and theelectrical signal generating at each different position for producing ananalog data output signal indicative of the spectral transmissiondensity characteristics of the liquid medium.

2. The method in accordance with claim lwherein the spectralcharacteristics of the radiation are varied at least between about 4,000to about 7,000 Angstrom units.

3. The method in accordance with claim 1 including the step ofgenerating a series of signals indicative of preselected spectralcharacteristics of the radiation incident upon the liquid medium.

4. The method in accordance with claim 3 including the step of recordingthe analog data output signals as a display of spectral transmissiondensity characteristics relative to the preselected spectralcharacteristics of the radiation incident upon the liquid medium.

5. The method in accordance withclaim 1 including the step of modulatingthe intensity of the source of radiation to maintain the generallyuniform amplitude of the reference signal.

6. A device for determining the spectral radiation transmissioncharacteristics and size-frequency distribution of very small particlesdispersed in a generally transparent, liquid medium, comprising:

a source of radiation having broad spectral characteristics;

means connected to a source of the liquid medium for moving anddirecting the latter in a flow path at a desired rate past said lightsource;

means arranged between the source of radiation and the flow path forcontinuously varying the spectral characteristics of a narrow bandwidthof the radiation incident on the liquid medium;

means responsive to the spectral characteristics of the narrow bandwidthof radiation for generating a generally continuous electrical referencesignal corresponding thereto;

means arranged in a plurality of different positions relative to theflow path so as to intercept the transmitted light scattered by thedispersed particles in the liquid medium for continuously detecting achange in intensity of the transmitted radiation as the spectralcharacteristics of the narrow bandwidth varies;

means responsive to the varying means for rendering each detecting meanssequentially operative for generating an electrical signal correspondingto the change in the transmitted radiation throughout the variation inthe narrow bandwidth spectral characteristics; and

means responsive to the electrical reference signal and the electricalsignal generated at each different position for differentiating the sameand for producing an analog data output signal indicative of thespectral transmission density characteristics of the liquid medium.

7. A device in accordance with claim 6 including means responsive to theanalog data output signal for producing a visual display of the spectraltransmission density characteristics of the liquid medium.

8. A device in accordance with claim 7 wherein the means for varying thespectral characteristics of the narrow bandwidth comprises a rotatable,spectrally variable filter having indicia for indicating one end of aspectral range and for indicating at least a number of divisions of therange.

9. A device in accordance with claim 8 wherein said filter is providedwith control indicia for initiating the electrical signal generatingmeans.

10. A device in accordance with claim 9 including means responsive tothe control indicia, the analog data output signal and the indicatingindicia for producing a visual display of preselected spectral valuesfor the filter relative to the spectral transmission densitycharacteristics of the liquid medium.

11. A device in accordance with claim 6 wherein the generating means forthe electrical signal includes a photosensitive detector arranged ineach different position and means for actuating the detectorssequentially.

12. A device in accordance with claim 6 including means responsive tothe electrical signal derived from the detecting means in each difierentand empirical information for producing a display of particlesizefrequency distribution in the liquid medium.

13. A device in accordance with claim 6 including means for modulatingthe intensity of the source of radiation to maintain the referencesignal at a generally uniform amplitude.

14. A device in accordance with claim 6 wherein said flow path comprisesa cell having transparent sides between which said liquid medium ismoved and through which the narrow bandwidth of continuously v ctralradiation is directed.

i5. d evice in accordance with claim 14 wherein the means for generatingthe electrical signal in each different position comprises a pluralityof photosensitive detectors spaced from and angularly disposed relativeto the cell and in the path of the radiation transmitted by the liquidmedium and scattered by the dispersed particles.

1. A method of determining the spectral radiation transmissioncharacteristics and size-frequency distribution of very small particlesdispersed in a generally transparent, liquid medium, the stepscomprising: flowing the liquid medium at a desired rate past a source ofradiation having bRoad spectral characteristics; continuously varyingthe spectral characteristics of a narrow bandwidth of the radiationincident on the liquid medium; generating an electrical reference signalof generally uniform amplitude from the radiation of narrow bandwidth;detecting continuously at a number of different positions relative tothe liquid medium a change in intensity of the transmitted radiation asthe spectral characteristics of the narrow bandwidth varies; generatingsequentially with respect to each different position an electricalsignal corresponding to the change in the transmitted radiationthroughout the variation in the narrow bandwidth spectralcharacteristics; and differentiating the electrical reference signal andthe electrical signal generating at each different position forproducing an analog data output signal indicative of the spectraltransmission density characteristics of the liquid medium.
 2. The methodin accordance with claim 1 wherein the spectral characteristics of theradiation are varied at least between about 4,000 to about 7,000Angstrom units.
 3. The method in accordance with claim 1 including thestep of generating a series of signals indicative of preselectedspectral characteristics of the radiation incident upon the liquidmedium.
 4. The method in accordance with claim 3 including the step ofrecording the analog data output signals as a display of spectraltransmission density characteristics relative to the preselectedspectral characteristics of the radiation incident upon the liquidmedium.
 5. The method in accordance with claim 1 including the step ofmodulating the intensity of the source of radiation to maintain thegenerally uniform amplitude of the reference signal.
 6. A device fordetermining the spectral radiation transmission characteristics andsize-frequency distribution of very small particles dispersed in agenerally transparent, liquid medium, comprising: a source of radiationhaving broad spectral characteristics; means connected to a source ofthe liquid medium for moving and directing the latter in a flow path ata desired rate past said light source; means arranged between the sourceof radiation and the flow path for continuously varying the spectralcharacteristics of a narrow bandwidth of the radiation incident on theliquid medium; means responsive to the spectral characteristics of thenarrow bandwidth of radiation for generating a generally continuouselectrical reference signal corresponding thereto; means arranged in aplurality of different positions relative to the flow path so as tointercept the transmitted light scattered by the dispersed particles inthe liquid medium for continuously detecting a change in intensity ofthe transmitted radiation as the spectral characteristics of the narrowbandwidth varies; means responsive to the varying means for renderingeach detecting means sequentially operative for generating an electricalsignal corresponding to the change in the transmitted radiationthroughout the variation in the narrow bandwidth spectralcharacteristics; and means responsive to the electrical reference signaland the electrical signal generated at each different position fordifferentiating the same and for producing an analog data output signalindicative of the spectral transmission density characteristics of theliquid medium.
 7. A device in accordance with claim 6 including meansresponsive to the analog data output signal for producing a visualdisplay of the spectral transmission density characteristics of theliquid medium.
 8. A device in accordance with claim 7 wherein the meansfor varying the spectral characteristics of the narrow bandwidthcomprises a rotatable, spectrally variable filter having indicia forindicating one end of a spectral range and for indicating at least anumber of divisions of the range.
 9. A device in accordance with claim 8wherein said filter is provided with Control indicia for initiating theelectrical signal generating means.
 10. A device in accordance withclaim 9 including means responsive to the control indicia, the analogdata output signal and the indicating indicia for producing a visualdisplay of preselected spectral values for the filter relative to thespectral transmission density characteristics of the liquid medium. 11.A device in accordance with claim 6 wherein the generating means for theelectrical signal includes a photosensitive detector arranged in eachdifferent position and means for actuating the detectors sequentially.12. A device in accordance with claim 6 including means responsive tothe electrical signal derived from the detecting means in each differentand empirical information for producing a display of particlesize-frequency distribution in the liquid medium.
 13. A device inaccordance with claim 6 including means for modulating the intensity ofthe source of radiation to maintain the reference signal at a generallyuniform amplitude.
 14. A device in accordance with claim 6 wherein saidflow path comprises a cell having transparent sides between which saidliquid medium is moved and through which the narrow bandwidth ofcontinuously varying spectral radiation is directed.
 15. A device inaccordance with claim 14 wherein the means for generating the electricalsignal in each different position comprises a plurality ofphotosensitive detectors spaced from and angularly disposed relative tothe cell and in the path of the radiation transmitted by the liquidmedium and scattered by the dispersed particles.